This invention relates to game balls, and, in particular, to a game ball with a molded plastic core formed from low density polyethylene and ethylene copolymer.
Historically, baseballs and softballs have been made by forming a spherical core of cork, kapok, or similar material which is bound together. A layer of windings of string or yarn is wrapped over the core and a leather cover is placed over the windings and stitched together.
In recent years, game balls, particularly softballs, have been made from molded plastic cores. For example, U.S. Pat. No. 4,149,720 describes a ball which includes a polyurethane core and a leather cover. U.S. Pat. No. 4,364,565 describes a ball which includes a spherical core made from ethylene vinyl acetate copolymer.
U.S. Pat. No. 4,610,071 describes a ball which includes a core which is molded from Nucrel (ethylene acid copolymer) and filled with polyurethane. A cover is sewn over the core.
Specifications for softballs have been issued by two governing organizations, the United States Slow-pitch Softball Association (USSSA) and the Amateur Softball Association (ASA). These specifications include the following requirements:
______________________________________ COEFFICIENT OF RESTITUTION ______________________________________ MAXRANGE USSSA .52 COR MAX 100% Low Density Polyethylene (LDPE) ASA .50 COR MAX 40/60% Ethylene Acid Copolymer (NUCREL)/LDPE +/-15% NUCREL MIDRANGE USSSA .48 COR MAX 50/50% NUCREL/LDPE +/-15% NUCREL ASA .47 COR MAX 30/70% NUCREL/LDPE +/-15% NUCREL CIRCUMFERENCE: 107/8 inches to 121/8 inches WEIGHT: Not less than 57/8 oz. nor more than 7 oz ______________________________________
The coefficient of restitution (COR) is extremely important because COR determines the liveliness of the ball. COR is measured by propelling a ball against a hard surface at 60 mph (88 fps) and measuring the rebound speed of the ball. COR is expressed in terms of the ratio and the rebound speed to 60 mph (88 fps).
Other qualities of softballs are important which are not included in the foregoing "official" specifications. These qualities include the sound of the ball when batted, texture of the ball, the "feel off the bat" or, the feel which the batter experiences at the moment of impact, flight consistency, and durability.